Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Design and performance simulation of a new solar continuous solid adsorption refrigeration and heating hybrid system

In this paper, the design of a new continuous solid adsorption refrigeration and heating hybrid system driven by solar energy was proposed, and its performance simulation and analysis were made under the normal working conditions. Some performance parameters of the system were obtained, and the effects of water mass in water tank on the system's COP_cooling, COP_heating etc. were discussed. The simulation indicated: the system could refrigerate continuously with such a design, and at the conditions of that the daily sun-radiation is 21.6 MJ, the mean ambient temperature is 29.9°C, the evaporating temperature is 5°C, the heat-collecting coefficient of upper bed η is 60%, and the heat-transfer coefficient between lower bed and ambient α is 2 W/m² K, by day a hybrid system of single combined bed could furnish 30 kg hot water of 47.8°C, and had a mean COP_cooling of 0.18, a mean COP_heating of 0.34, a continuous mean SCP_a of 17.6 W/kg, a continuous mean SCP_c of 87.8 W/m², and a continuous mean SHP_c of 165.9 W/m²; and at night it had a cooling capacity of 0.26 MJ/kg of adsorbent, and a cooling capacity of 1.3 MJ/m² of heat-collecting area.     

Study of a novel solar adsorption cooling system and a solar absorption cooling system with new CPC collectors

One two-phase thermo-syphon silica gel-water solar adsorption chiller and LiBr-H₂O absorption chiller with new medium CPC (Compound Parabolic Concentrator) solar collectors were investigated. The reliability of adsorption chiller can be improved, because there is only one vacuum valve in this innovative design. Medium temperature evacuated-tube CPC solar collectors were firstly utilized in the LiBr-H₂O air conditioning system. The former system was applied in north of China at Latitude 37.45° (Dezhou city, China), the latter system was applied at Latitude 36.65° (Jinan city, China). Experimental results showed that the adsorption chiller can be powered by 55 °C of hot water. The adsorption chiller can provide 15 °C of chilled water from 9:30 to 17:00, the average solar COP (COPs) of the system is 0.16. In the absorption cooling system, the efficiency of the medium temperature evacuated-tube CPC solar collector can reach 0.5 when the hot water temperature is 125 °C. The absorption chiller can provide 15 °C of chilled water from 11:00 to 15:30, and the average solar COPs of absorption system is 0.19.     

Simulation of an adsorption solar cooling system

A more realistic theoretical simulation model for a tubular solar adsorption refrigerating system using activated carbon-methanol (AC/M) pair has been introduced. The mathematical model represents the heat and mass transfer inside the adsorption bed, the condenser, and the evaporator. The simulation technique takes into account the variations of ambient temperature and solar radiation along the day. Furthermore, the local pressure, and local thermal conductivity variations in space and time inside the tubular reactor are investigated as well. A C++ computer program is written to solve the proposed numerical model using the finite difference method. The developed program covers the operations of all the system components along the cycle time. The performance of the tubular reactor, the condenser, and the evaporator has been discussed. Time allocation chart and switching operations for the solar refrigeration system processes are illustrated as well. The case studied has a 1 m² surface area solar flat plate collector integrated with a 20 stainless steel tubes containing the AC/M pair and each tube has a 5 cm outer diameter. In addition, the condenser pressure is set to 54.2 kpa. It has been found that, the solar coefficient of performance and the specific cooling power of the system are 0.211 and 2.326 respectively. In addition, the pressure distribution inside the adsorption bed has been found nearly uniform and varying only with time. Furthermore, the AC/M thermal conductivity is shown to be constant in both space and time.     

吸收式太阳能制冷空调系统的性能分析

介绍了一个吸收式太阳能制冷空调系统,该系统由太阳能集热器、吸收式制冷机组、辅助加热装置、智能控制器等部分组成。文章对该系统一年多的运行数据进行了采集和整理,针对夏季与冬季的典型工况进行了性能分析,并与常规空调进行了经济性对比,分析其投资回收期。     

Performance of a solar LiBr-water absorption refrigeration system

This paper describes the performance of a nonstorage, continuous, solar operated absorption refrigeration cycle. The solar collectors and the refrigeration experimental unit are manufactured locally. The results presented in this paper are based on the operation of the unit when there was enough solar energy to power its generator. This amounted to 4–5 h per day during the months of August and September in Amman, Jordan. These results include the variation of both the generator and evaporator temperatures during the test periods, the performance of the unit as a function of these temperatures and available solar intensity, and the ideal performance of the unit based on the measured temperatures. The maximum ideal coefficient of performance was determined as being equal to 1.6, while the peak actual coefficient of performance was determined to be equal to 0.55. These values are very close to published values.     

Performance of a self-regulating solar multistage flash desalination system

A 10,000-1 solar multistage flash desalination system was designed and tested at Kuwait Institute for Scientific Research. The system consisted of a 220 m² solar line-concentrating collector field, 7000-1 thermal storage tank and a self-regulating 12-stage multistage flash desalination subsystem. The collector field, equipped with closed-loop tracking system, was installed with individual troughs oriented in the north-south direction. The thermal storage subsystem was useful in leveling off the thermal energy supply and allowing the production of desalinated water to continue during periods of low radiation and nighttime. The self-regulating capability of the desalination subsystem allowed for the adjustment of the flow rate and pressure, in the various stages of the desalination subsystem, according to the relative difference between the hot brine and input sea water temperatures. This allowed for maintaining a relatively high overall efficiency.     

Computational study of a novel continuous solar adsorption chiller: performance prediction and adsorbent selection

A novel solar adsorption chiller intended for domestic use is presented. The chiller can be integrated with existing solar systems based on flat plate collectors, and, contrary to commercial chillers, it operates continuously. A detailed analysis of both the simple and the heat‐integrated cycle is carried out so as to select the optimal adsorbent and operating conditions. The employed integral thermodynamic model takes into account the inert masses that limit the performance of the chiller, such as the metal frame, the thermo‐fluid, and the non‐adsorbed steam, by introducing heat capacity effects. Given the adsorption equilibrium data, the energy balances, the performance, and the useful thermal loads of the system can be calculated at any operating conditions. The results indicate that silica gel Type A is a more efficient adsorbent compared to silica gel Type RD or Type 3A. Furthermore, the total porosity has a slight effect on system performance, while optimal operation can be achieved when the condenser temperature is less than 326 K and the evaporator temperature greater than 280 K. Copyright © 2006 John Wiley & Sons, Ltd.     

Heat-exchanger design and switching-frequency effects on the performance of a continuous type solar adsorption chiller

A transient one-dimensional model, capable of describing the performance of a newly-introduced adsorption chiller with continuous operation, is developed. Since the cycle time and the switching frequency have a great influence on chiller performance, a non-dimensional switching frequency is introduced and a systematic parametric study is carried out in order to determine regions of optimal operation. An optimization based on the thermodynamic efficiency yields a lower switching frequency than an optimization based on the maximum cooling capacity. In addition, the effect of the heat-exchanger design parameters on system performance is explored. An increase of either the bed’s Fourier number or the thermofluid’s Nusselt number has a positive effect on both COP and cooling capacity. An improvement of system performance can also be achieved by decreasing either the thermofluid’s Fourier number or the bed’s Biot number. Finally, the effect of space velocity of the thermofluid exhibits the most interesting behavior; an increase of the space velocity has a positive effect on cooling capacity and a negative effect on COP.     

A continuous cycle solar thermal refrigeration system

A Platen-Munters refrigerator has been modified to operate with heat collected from solar energy conversion in a solar thermal continuous cycle refrigeration system. The results show that temperatures as low as −19°C are routinely achievable. The incorporation of a solar thermal battery can increase the refrigeration period to as long as 10 hours to provide cooling power for as long as 36 hours.     

Thermal design of parabolic solar concentrator adsorption refrigeration system

This study presents a thermal design of solar-powered adsorption refrigeration with the type of activated carbon-methanol pair. The designed module consists of an evacuated glass tube equipped with a parabolic solar concentrator as generator, sorption bed, evaporator, and condenser units. A thermodynamic design procedure and a mathematical model of a steady state system with activated carbon refrigerator have been developed. The adsorber is heated by solar energy collected by a parabolic solar concentrator. The temperature of the working pair in the adsorber, the amount of methanol leaving and reabsorb bed, and the refrigerated box was estimated. An optimize design of the system to achieve higher cycle COP was presented. Maximum cycle COP = 0.576 and COP_net = 0.375 with T _max reached 157.8°C, T _B = 57.5°C, M _ac = 0.907 kg, and the concentration of methanoldesorped equal to 0.206 kg/kg _ac .     

Performance of a multi-functional direct-expansion solar assisted heat pump system

This paper reports on the long-term performance of a direct-expansion solar assisted heat pump (DX-SAHP) system for domestic use, which can offer space heating in winter, air conditioning in summer and hot water during the whole year. The system employs a bare flat-plate collector array with a surface area of 10.5 m², a variable speed compressor, a storage tank with a total volume of 1 m³ and radiant floor heating unit. The performance under different operation modes is presented and analyzed in detail. For space-heating-only mode, the daily-averaged heat pump COP varied from 2.6 to 3.3, while the system COP ranged from 2.1 to 2.7. For water-heating-only mode, the DX-SAHP system could supply 200 l or 1000 l hot water daily, with the final temperature of about 50 °C, under various weather conditions in Shanghai, China. For space-cooling-only mode, the compressor operates only at night to take advantage of a utility’s off-peak electrical rates by chilling water in the thermal storage tank for the daytime air-conditioning. It shows that, the multi-functional DX-SAHP system could guarantee a long-term operation under very different weather conditions and relatively low running cost for a whole year.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in the ground; the ground is covered with a glazed thermal trap. The heat is extracted by means of a flow of liquid in such a way that the collection temperature remains constant. An expression has been derived for the periodic rate at which heat can be retrieved to keep the collection temperature constant. Numerical calculations for a typical cold day in New Delhi predict that, for a collection temperature of 30°C and for a trap thickness of 0·03 m, the maximum possible integrated collection efficiency is 25·7 per cent.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in a mass of sand; the sand is covered with a glazed thermal trap. The heat can be extracted by flow of fluid in the pipes at a constant flow rate. An expression has been derived for the periodic rate at which useful heat can be collected, keeping the flow rate constant. Numerical calculations for a typical cold day in Delhi predict that the collection efficiency of the system is about 50% for flow rate of 10 kg/h. The efficiency increases with thermal trap thickness and with flow rate.     

Performance analysis of solar space heating system

The paper deals with solar space heating for a school during winter. The system, which is an extension of Solar Vapour Absorption Cooling Project, consists of coupling the solar collector field, the air handling units and the thermal energy reservoir in a single circuit with one pump. The analysis of the system has been done to describe the temperature history of the hot water reservoir as a function of two non-dimensional parameters. These parameters, which are functions of environmental and air handling unit characteristics, dictate the net thermal energy inflow or outflow from the reservoir. The problem solution has been obtained in a generalized form and predicted results are shown in dimensional form for the basic data chosen from an actual on-going project.     

Performance of a solar air composite heat source heat pump system

For the shortcoming of air source heat pump in heating condition, a composite heat exchanger was designed which integrates fin tube and tube heat exchanger, and it can achieve synchronous and composite heat exchange in one heat exchanger between working fluids, gaseous and liquid heat source. With the above composite heat exchanger as the core component, the Solar Air Composite Heat Source Heat Pump System (SACHP) was developed which has three working modes, including single solar heat source mode, single air heat source mode and solar air dual heat sources mode. A SACHP experiment table was established and conducted a comprehensive experimental study of three working modes of this system in the standard enthalpy difference laboratory. The results show that when the ambient temperature was −15 °C, compared to the single air heat source mode, the dual heat source mode increased 62% in heat capacity and 59% in COP; when the temperature difference of combined heat transfer was 5 °C, compared to the single air heat source mode, the dual heat source mode increased 51% in heat capacity and 49% in COP. Experimental results demonstrate that the application of the solar air composite heat pump technology can accelerate the application process of the solar heat pump in air conditioners for buildings.     

太阳能低温储粮吸附制冷系统

低温储粮是一种具有广阔应用前景及实用价值的科学储粮方法。从降低能耗的角度出发,设计并建造了一种用于低温储粮的太阳能吸附制冷系统,测试分析了系统的热性能。试验测试结果表明：通过合理配置集热器面积,太阳能制冷系统能够满足低温储粮对冷负荷的需求。     

Performance equations for a closed-loop water-heating system using a solar air-heating collector

Performance equations of the HWB type have been derived for a closed-loop water-heating system using a solar air collector. Explicit expressions are given for the collector efficiency and overall heat-loss coefficient. The utility of the performance equations has been established with the help of an experiment in which water is recirculated from a storage tank through the air/water heat exchanger in a closed-loop cycle.     

Preliminary observations of a continuous flow solar disinfection system for a rural community in Kenya

This project involves the design and installation of a continuous flow reactor which uses solar radiation, an abundant resource in most sub-Saharan countries, to disinfect water supplies. The system was installed at a rural village in Kenya to disinfect surface water collected at a recently constructed micro-dam. The solar reactor uses CPC reflectors which reflect both direct and diffuse solar radiation onto clear pipes through which the requisite water supply flows. The reaction kinetics for the full-scale design had been determined on a variety of pathogenic micro-organisms under both artificial and real sunlight in controlled conditions. The community fully participated in the planning, installation and subsequent operation of the system. The preliminary water quality results indicate that the system is providing a safe source of water for the community. The installation, commissioning and initial use of the system highlights the critical need for community involvement and approval if such interventions are going to be successful in rural areas, alongside the requirements for strategic and technical support.     

Performance of a solar chimney

A mathematical model of a solar chimney was proposed in order to predict its performance under varying ambient and geometrical features. Steady state heat transfer equations were set up using a thermal resistance network and solved using matrix inversion. Existing correlations of heat transfer coefficients were utilised. Property values for the air flow in the duct were based on mean bulk or film temperatures. The performance of the chimney was evaluated by predicting the temperatures of the glass glazing and the heat-absorbing wall and also the temperature and velocity of the induced air flow in the chimney. The effects of air gap and solar radiation intensity on the performance of different chimneys were investigated. In order to verify the theoretical model, experiments were conducted on a 2 m high×0.45 m wide physical model with air gaps of 0.1, 0.2 and 0.3 m. Experiments were carried out outdoors on the roof and the experimental model exposed to both direct and diffuse solar radiation. Air velocities between 0.25 m s⁻¹ and 0.39  m s⁻¹ for radiation intensity up to 650 W m⁻² were obtained. No reverse air flow circulation was observed even at the large gap of 0.3 m.